1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to a wireless local area network (WLAN) which includes a plurality of stations, and more particularly, to preventing a plurality of stations in a WLAN from colliding with each other when attempting to access a medium.
2. Description of the Related Art
In a WLAN, Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)-based Medium Access Control (MAC) methods are widely used. CSMA/CA is a type of data transmission method in which, even when no data is currently being transmitted via a network cable, a signal for determining whether data is currently being transmitted via the network cable is transmitted and data is transmitted only after the signal is determined to have been successfully transmitted without collision.
According to CSMA/CA, a first station detects a sub-carrier which indicates whether a station is currently transmitting data. If a station is currently transmitting data, the first station stands by for a predetermined amount of time, determines whether a sub-carrier is being transmitted by another station, and begins to transmit data if a sub-carrier is not currently being transmitted.
According to CSMA/CA, both a physical carrier sensing method and a virtual carrier sensing method are used simultaneously. The physical carrier sensing method is a carrier sensing method in which a physical layer (PHY) determines whether a power of higher than a predefined value has been received and notifies an MAC layer whether a medium is currently busy or idle based on the results of the determination, and the virtual carrier sensing method is a carrier sensing method in which, if an MAC protocol data unit (MPDU) can be properly extracted from a received Physical Layer Convergence Procedure (PLCP) protocol data unit (PPDU), stations interpret one of a plurality of header fields of the MPDU, i.e., a duration/identifier field of the MPDU, and determine whether a medium is currently busy based on the results of the interpretation. Stations use both the physical carrier sensing method and the virtual carrier sensing method to determine whether a medium is currently busy and do not attempt to access the medium if the medium is determined to be busy.
Referring to FIG. 1A, an MAC header of a data frame which can be transmitted via a typical 802.11 WLAN includes duration information which specifies the time required to receive an acknowledgement (ACK) frame in return for the data frame after the transmission of the data frame. A plurality of stations which receive the data frame interpret the MAC header of the data frame and do not attempt to access a medium during a predetermined time period specified in the MAC header of the data frame. Therefore, the stations can be prevented from colliding with each other. Due to the characteristics of a WLAN, all of a plurality of stations in a WLAN can hear frames regardless of whether the frames are destined for only one of the stations.
FIG. 1B is a diagram illustrating the format of a data frame which is used in a typical 802.11a network. Referring to FIG. 1B, a signal field of the 802.11a frame includes rate information (RATE) and length information (LENGTH). Thus, duration information of the data frame can be obtained by analyzing the rate information and the length information of the data frame. Therefore, the virtual carrier sensing method can be realized.
The virtual carrier sensing method can be effectively applied to CSMA/CA only when an MPDU/PHY service data unit (PSDU) can be interpreted properly without any errors, i.e., only when the value of an MAC header of a frame can be read out properly.
When errors occur due to an unstable channel state during the transmission of a frame at a high transmission rate by a transmitting station, or when a receiving station cannot properly handle the high transmission rate, a received MPDU/PSDU cannot be interpreted properly. In this case, virtual carrier sensing cannot be used, and thus, the performance of CSMA/CA decreases. Therefore, when a sending station transmits a data frame having a high throughput (HT) format in a WLAN where a plurality of 802.11a/b/g legacy stations and a plurality of HT stations such as multi-input multi-output (MIMO) stations which have better data transmission capabilities than the 802.11a/b/g legacy stations coexist, the 802.11a/b/g legacy stations cannot perform a virtual carrier sensing operation because they cannot interpret a frame which has the HT format. This problem also arises when some of the HT stations in the WLAN use channel bonding and thus use channel bandwidths which are different from the other HT stations.